The invention refers to a method for receiving and processing AltBOC-modulated satellite navigation signals transmitted in the two partial bands E5a and E5b of the so-called E5 frequency band are received using a common antenna and thereafter are processed separately in an analog manner in the HF front-end in two physically different incoming signal paths, each performing a down conversion, for the partial bands E5a and E5b and then digitized using analog-to-digital conversion and coherently summed to form a complete digital E5 band signal, which is fed to a digital signal processor, in which code acquisition and code tracking are performed using PRN reference code sequences produced in the receiver and tracking of the carrier phase is performed and raw data for the final navigation calculation are determined therefrom.
The invention further refers to a satellite navigation receiver for carrying out the method.
It is known that receivers implemented in global satellite navigation systems (GNSSs), such as GPS (Global Positioning System), for instance, determine their position on the basis of received signals emitted from satellites that are a part of a global satellite constellation, e.g. GPS satellites. The satellites that belong to the GPS satellite constellation emit their signals at two carrier frequencies L1 and L2, wherein the carrier L1 has a frequency of 1575.42 MHz and the carrier L2 has a frequency of 1227.60 MHz.
Each carrier is modulated with at least one pseudorandom binary code sequence PRN (pseudorandom noise) that is formed by a seemingly random, periodically repeating sequence of zeroes and ones. The PRN sequences are referred to as ranging codes since they allow an estimation of the distances (“ranges”) between a receiver and a satellite. Each satellite uses a PRN code sequence of its own, whereby the receiver can associate the received signal to the satellite that has transmitted the same. The receiver calculates the difference between the moment at which the satellite has emitted the signal, which information is included in the signal itself, and the moment at which the receiver itself has received the signal.
Based on the temporal difference, the receiver calculates its own distance (pseudorange) from the satellite. The receiver can calculate its own global position from the determined distances to at least four satellites. For the purpose of determining the temporal difference between the above-mentioned transmission time and the above-mentioned receiving time of the signal, the receiver synchronizes a locally generated PRN reference code sequence with the PRN code sequence included in the received signal.
In this manner, the receiver determines the amount of time deviation of the locally generated PRN reference code sequence with respect to the satellite time and calculates the distance. The synchronization operations comprise acquiring the PRN code sequence of the satellite and the tracking thereof (code tracking). In addition, the receiver usually tracks the phase of the carrier that is used by the satellite to transmit the PRN code sequences and the navigation data (phase tracking).
At present, a new satellite navigation system by the name of Galileo is realized which provides a very high precision and various services. It operates in the principal frequency ranges, namely L1 (1559-1591 MHz), E6 (1260-1300 MHz) and E5 (1164-1214 MHz). The so-called E5 band comprises, on the one hand, a plurality of partial signals that use only a respective one of two partial bands E5a (1164-1191 MHz) or E5b (1191-1214 MHz) and, on the other hand, a signal that uses the full E5 bandwidth. FIG. 1 illustrates the Galileo frequency spectrum in detail.
Details of the Galileo system and related receiving methods as well as of the receivers are described for example in the essay by M. Hollreiser: “Galileo Receivers—Challenges and Performance”, 12th GAAS Symposium, Amsterdam, 2004, pp. 515-518, and in EP 2 012 488 B1.
As far as the E5 band signal is concerned in particular, the satellites of the new Galileo satellite navigation system transmit the signals in the partial band E5a (centre frequency 1176.45 MHz) and in the partial band E5b (centre frequency 1207.14 MHz) in the form of a composite signal with a centre frequency of 1191.795 MHz using a modulation format generally known by the name AltBOC (Alternate Binary Offset Carrier).
The large bandwidth of the E5 signal of about 52 MHz does indeed allow a very precise localizing, however, it places high demands on the development of receivers. Due to the large bandwidth a very great effort in digital signal processing arises. Moreover, the large receiving bandwidth increases the susceptibility of the receiver to faults. Therefore, one strives to process the two partial bands E5a and E5b separately. This means that the two partial bands E5a and E5b are processed by two physically different signal paths.
However, due to this separate processing of the two partial bands, valuable information about the relation between the two signals is lost. From the article by N. Martin, H. Guichon, M. Revol, M. Hollreiser, J. De Maestro: “Architecture of the GAII LEO TUS receiver for coherent AltBOC tracking”, 3,d CNES-ESA Workshop on GNSS Signals and Signal Processing, 21&22 Apr. 2008, IAS (INSTITUT AERO SPATIAL), Toulouse, France, a technology for solving this problem is known, wherein the separately received Galileo E5a and E5b partial band signals are used to correct the different characteristics of the two HF-front end signal paths on the digital level.
Here, a coherent AltBOC processing is realized on the same hardware demodulation architecture as used for the independent processing, however, with a coherent summing of the two components on a digital software level. The disadvantage of this known method is, however, that the received Galileo signals in the partial band E5a and E5b are disturbed by a variety of influences, such as, for example, multi path propagation, ionosphere errors and interferences. These influences are highly frequency-selective and can significantly compromise the calibration.
FIG. 2 illustrates a block diagram of a Galileo signal receiver operating according to this known method in the frequency band E5. The satellite navigation signals received through an antenna 1 are first amplified in a low-noise pre-amplifier (LNA) 2 and are then divided by means of two band filters 3 and 4 onto the partial band E5a/L5 and the partial band E5b into two independent physically separate incoming signal paths. In each of the two incoming signal paths, the filtered signals are then amplified in a HF amplifier 5 and 6, respectively, and are thereafter, while being filtered by a polyphase filter 10, converted to the intermediate frequency range by means of a down-converter 7 and 8, respectively, operated via a local reference oscillator 9.
The received signals converted to the intermediate frequency range are then amplified in each of the two incoming signal paths by means of an intermediate frequency amplifier 12 and 13, respectively, and are thereafter supplied to a VGA amplifier 14 and 15, respectively, with variable gain that is set by a digital signal processor through an AGe control loop, which digital signal processor could be configured as a FPGA (Field Programmable Gate Array) A/D board 16 and also includes the analog-to-digital converters for the digitization of the two analog received signals. In the FPGA A/D board 16, a coherent summing of the two digitized signals is performed on the digital level.
The digital signal processor comprises means for acquiring codes and for tracking codes using a PRN reference code sequence generator provided in the receiver, as well as a means for tracking the carrier phase. The raw data determined in the digital signal processor are supplied to a means for final navigation calculation. The IF received signal amplified in the VGA amplifier 14 and 15, respectively, is further passed through an anti-aliasing filter 17 and 18, respectively, in each incoming signal path, before it is subjected to analog-to-digital conversion in the FPGA A/D board 16.